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Superconducting Fault Current Limiter
SFCL
OVERVIEW
•
Introduction.
•
Why SFCL?
•
Modelling of SFCL.
•
Doubly-Fed Induction Generator
(DFIG) Characteristics.
INTRODUCTION

With the increase of electricity demand and change of concerning
environment, the capabilities of renewable energy generation
systems are being expanded.

Renewable energy sources considered as clean and prospective
energy sources of the future world.
 12% of world’s electricity is generated from wind power.
 The wind-turbine generation system (WTGs) is a representative
renewable energy system.
 Does not cause pollution problem.
 Lowest maintenance cost.
Why SFCL?
Superconducting Fault Current Limiter
(SFCL):
 Reduces the peak value of fault current.
 Improves the transient stability of the power
system.
 Provides the system effective damping for low-
frequency oscillations.
Connection of SFCL to an electric power
grid:



Optimal place to install the SFCL.
Optimal resistive value of the SFCL
occurred in series with a transmission line
during a short circuit fault.
Potential protection-coordination problem
with other existing protective devices such
as circuit breakers.
Modelling Of SFCL
•
•
Modelling of a resistive (non-inductive winding)
SFCL.
Consists of:
 Stabilizer
resistance of the n-th unit, Rns.
 Superconductor
resistance of the n-th unit,
Rnc(t) connected with Rns in parallel.
 Coil
inductance of the n-th unit, Ln.
Structure of a resistive SFCL:
•
Normal steady state condition, the values of Rnc(t)
and Rns(t) are normally zero.
•
Total resistance of parallel connection becomes
zero in steady state condition.
•
Total resistance (Rsfcl) of the SFCL during fault
depends on total number of units in series.
•
Value of Ln determined by the wound coils.
Expression to describe quenching and
recovery characteristics:
Rm - Maximum resistance of superconducting
coil in the normal state
Tsc - Time constant of transition from
superconducting state to normal state
(Tsc = 1ms)
Doubly-Fed Induction
Generator (DFIG)
•
Induction generator widely used as wind generators
 Brushless
 Low
and rugged construction.
cost.
 Maintenance
free.
•
Two types of wind generator topologies :
 Fixed
speed wind generator.
 Variable
speed wind generator.
•
DFIG mostly used as variable speed wind
generator.
•
DFIG wind turbines are based on wound-rotor
induction machines where rotor circuit is fed
through back-to-back voltage source converters.
General control scheme for the DFIG
system in Power Factory:
•
DFIG generator model is a built-in model which
integrates the induction machine and rotor-side
converter (RSC).
•
DFIG and RSC modelled in rotor reference frame
(RRF) rotating at generator speed.
•
RSC controller operates in a stator flux-oriented
reference frame (SFRF) rotating at synchronous
speed.
•
RSC control modifies the active (P) and reactive (Q)
Equations for modelling DFIG :
- flux linkage
- base angular speed
- stator electrical angular speed
Active and reactive powers transferred to stator, can
be computed by :
System Configurations
For Two Case Studies
Single-Machine Infinite Bus System
(SMIB)
• System consists of one DFIG and a transmission
system connected to an infinite bus.
• SFCL is located between the DFIG and the infinite bus.
• Power scale of wind farm by the DFIG is 9MW.
• Capacitive bank is used.
• Wind speed is fixed as 10 m/s.
• Proper value of SFCL will guarantee enough time
intervals for adequate protective coordination
between multiple OCRs.
• Too small value of Rsfcl :
 SFCL cannot provide desirable damping
performance for low- frequency oscillations during
a fault.
IEEE Benchmarked four-machine
two-area test system
• System consists of AREA 1 and AREA 2 linked
together by two 230kv transmission line of lengths
220km.
• Each area equipped with two identical synchronous
generators of 20 kv/900 MVA.
• Each generator produces active power of 700 MW.
IEEE benchmark four machine two-area system with
the WTGs
• SFCL located next to Wind Turbine Generation
System (WTGs) based on DFIG of 9 MW
connected to bus 4 in AREA 2.
• A 100 ms three-phase short-circuit current applied
to bus 4 at 0.1s, performance of SFCL to improve
low-frequency oscillation damping is evaluated.
• Wind speed is fixed as 10 m/s.
• Resistance of Rsfcl is 30 in normal stage of
SFCL.
SFCL Performance
Case study on SMIB system
• SMIB system without SFCL :
 Rotor speed increased by the fault and settles slowly after
clearing the fault.
 Terminal voltage drops from 1 pu to 0.4 pu
 Output active power response shows large oscillations from
2 pu to 1.2 pu.
 a-phase current of WTGs increases to 2.34 pu at its
maximum.
• SMIB system with SFCL :
 Variations of output active power dramatically reduced and
rapidly damped.
 Power scale of WTGs can be extended by the SFCL.
 a-phase current almost same as that of steady state
operation.
Response of a-phase current of WTGs
Case study on Multi-Machine Power
system
• System without SFCL :
 Rotor speed increases up to 1.0035 pu at its maximum.
 Takes very long time to restore its original steady-state
value.
 Transient variations for the terminal voltage and active
power responses of WTGs are more severe .
 Peak value of a-phase current increases up to 2.2 pu after
the fault.
• System with SFCL :
 Peak value of a-phase current is 0.51 pu.
 SFCL extends the scale of WTGs in multi-machine power
systems.
Response of a-phase current of WTGs
Effect Of WTGs On
Power
• WTGs increase
the level of Grid
short-circuit current if there
are no any protective devices.
Response of a-phase current flowing from bus 4 for the second
• Peak value of the fault current from 0.1 s to 0.2 s
without WTGs is less than that with WTGs for both
case studies.
• System more sensitive to fault.
Effect Of SFCL On
Overcurrent Relay Operation
• Overcurrent relay (OCR) is a protective relay.
• OCRs must guarantee :
 Fast operation
 Reliability
 Selectivity
• OCRs has inverse time characteristics.
• OCRs has specific rated short-circuit current.
(a) Distribution system with the DG, the SFCL, and two
OCRs
(b) Operation characteristics of OCR-1 and OCR-2
according to the alteration of the power system
•
Multiple OCRs have required coordination time
in steady-state condition.
•
If short circuit current increases over a rated
value of OCR-2, results in its malfunction.
To solve the problem :
 SFCL is applied with the distribution
generation.
 SFCL reduce the level of short-circuit current
during a fault.
CONCLUSION
•
SFCL provides quick system protection during a
severe fault.
•
Effectiveness of SFCL as protective device is
verified with several case studies.
•
Simulation results showed that SFCL reduces the
level of short-circuit current which is increased by
WTGs.
REFERENCES
 S. M. Muyeen, R. Takahashi, M. H. Ali, T. Murata, and J. Tamura,
“Transient stability augmentation of power system including wind
farms by using ECS,” IEEE Trans. Power Syst., vol. 23, no. 3, pp.
1179–1187, Aug. 2008.
 M. Kayikci and J. V. Milanovic, “Assessing transient response of
DFIG-based wind plants-the influence of model simplifications and
parameters,” IEEE Trans. Power Syst., vol. 23, no. 2, pp. 545–554,
May 2008.
 L. Ye, M. Majoros, T. Coombs, and A. M. Campbell, “System studies
of the superconducting fault current limiter in electrical distribution
grid,” IEEE Trans. Appl. Supercond., vol. 17, no. 1, pp. 2339–2342,
Jun. 2007.
 B. C. Sung, D. K. Park, J.-W. Park, and T. K. Ko, “Study on optimal
location of a resistive SFCL applied to an electric power grid,” IEEE
Trans. Appl. Supercond., vol. 19, no. 3, pp. 2048–2052, Jun. 2009.